1. Field of the Invention
The present invention concerns non-volatile electrically programmable and erasable memories.
The present invention particularly concerns EEPROM type memories. However, the present invention is applicable to memories implemented in different technologies in which similar problems are posed.
2. Discussion of the Related Art
In order to register data, i.e. store data, EEPROM memories store electrical charge on a floating gate of a transistor. Floating gate transistors also have an associated control gate. The floating gate transistor may be made to conduct by the application of a read voltage that is applied to the control gate. According to the quantity of charge stored on the floating gate, the conduction threshold of the transistor varies. It is this variation of threshold voltage that allows one to determine whether the transistor is erased or programmed.
As a consequence, the reading of data is carried out by applying a determined reference potential to the control gate. This reference potential is carefully chosen such that it is midway between the conduction threshold voltage of a programmed transistor and the conduction threshold voltage of an erased transistor. Therefore, if the transistor is programmed it will be made to conduct by the application of the reference potential to its floating gate; if it is erased it will not be made to conduct.
However, the quantity of electrical charge stored on the floating gate is difficult to control. Additionally, the time that the charge remains stored on the floating gate is not well controlled. Furthermore, the programming and erasing conduction threshold voltages are modified as a function of time. These conduction threshold voltages are notably modified as a function of the number of write, i.e. programming, or erase cycles that are executed by the memory.
It may happen that, because of the aging of the information, i.e. charge, contained in a memory cell, the information is such that the read circuit (that uses the reference potential) is not capable of determining with a sufficient amount of certainty whether a memory cell is erased or programmed. The possible causes of the aging of information will be discussed further on in this text.